In recent years, the development of the wireless communication industry is vigorous. The wireless communication devices, for example, cell phones or PDAs, have become an indispensable commodity for people. Antenna devices generally play an important role for transmitting and receiving wireless signals in a wireless communication device. Therefore, the operating characteristics of the antenna device have a direct impact on the transmission and receiving quality for the wireless communication device.
Generally, the antenna device of the portable wireless device is roughly classified into two categories, including the external type antenna and embedded type antenna. The external type antenna is commonly shaped as a helical antenna, and the embedded type antenna is commonly shaped as a planar inverted-F antenna (PIFA). The helical antenna is exposed to the exterior of the casing of the wireless communication device and is prone to be damaged. Thus, the helical antenna usually bears a poor communication quality. A planar inverted-F antenna has a simple structure and a small size and is easily to be integrated with electronic circuits. Nowadays, planar inverted-F antenna has been widely employed in a variety of electronic devices.
Typically, a well-designed antenna device is required to have a low return loss and a high operating bandwidth. In order to allow the user of the wireless communication device to receive wireless signals with great convenience and high quality, the current wireless communication devices have been enhanced by increasing the number of antenna devices or enlarge the antenna device to allow the wireless communication device to transmit and receive wireless signals with a larger bandwidth or multiple frequency bands. However, with the integration of circuit elements and the miniaturization of the wireless communication device, the conventional design method has been outdated.
In order to allow the wireless communication device to increase the number of antenna devices in the limited receiving space so as to transmit and receive wireless signals with a larger bandwidth and a better transmission quality and performance, the structure of the antenna device has been modified. Referring to FIG. 1, the structure of a conventional multiple frequency band antenna is shown. As shown in FIG. 1, the conventional multiple frequency band antenna 1 is made up of a planar inverted-F antenna having a first radiating element 11, a second radiating element 12, and a parasitic element 13. Also, a feeding point 14 and a first ground terminal 15 are disposed at one side of the distal region of the second radiating element 12, and a second ground terminal 16 is disposed at one side of the distal region of the parasitic element 13. The distal region of the first radiating element 11 and the distal region of the second radiating element 12 are connected with each other, and the parasitic element 13 is separated from the first radiating element 11 and the second radiating element 12 and approximate to the first radiating element 11. The multiple frequency band antenna 1 is adapted for dual frequency band applications, where the low frequency band is the frequency band located at 880-960 MHz of the GSM system, and the high frequency band is the frequency band located at 1920-2170 MHz of the WCDMA system.
Referring to FIG. 1 again, the feeding point 14 can feed the RF signals to be transmitted by the RF circuits (not shown) to the multiple frequency band antenna 1. Certainly, the feeding point 14 can feed the RF signal sensed by the multiple frequency band antenna 1 to the RF circuits. The first radiating element 11 is shaped like a right hand square bracket “]” and has a longer path length compared with the second radiating element 12, thereby forming a resonant mode to transmit and receive wireless signals in a low frequency band located at, for example, 880-960 MHz of GSM system. The second radiating element 12 is shaped like the character “L”, and the linear segments of the second radiating element 12 that are not connected with the first radiating element 11 are located in the gap between two opposing linear segments of the first radiating element 11. Consequently, the second radiating element 12 has a shorter path length compared with the first radiating element 11, and thus the second radiating element 12 can form a resonant mode to transmit and receive wireless signals in a high frequency band located at, for example, 1920-2170 MHz of the WCDMA system. The parasitic element 13 is configured to increase the bandwidth of the high frequency band.
Referring to FIG. 2, the standing-wave ratio versus frequency relationship of the multiple frequency band antenna of FIG. 1 is shown. As shown in FIG. 2, the longitudinal axis represents the standing-wave ratio (SWR) of the multiple frequency band antenna 1 that shows a linear relationship with the gain value of the return loss. Also, the standing-wave ratio can be converted into the gain value of the return loss through computations. It is to be noted that the standing-wave ratio will vary with the frequency. Generally, if the antenna 1 has a standing-wave ratio below 3 under a frequency band, it indicates that the antenna performs well under that frequency band. Hence, it can be understood from FIG. 2 that the multiple frequency band antenna 1 of FIG. 1 is adapted for the low frequency band located at 880-960 MHz of the GSM system, and for the high-frequency band located at 1920-2170 MHz of the WCDMA system.
In addition to the aforementioned antenna 1, the Taiwanese Patent Application No. 092119341 entitled “multiple frequency band antenna for cell phone” also discloses another antenna structure for use with dual frequency band applications, where the low frequency band is located at the frequency band of the GSM system and the high frequency band is located at the frequency band of personal communication services (PCS) system. However, the contemporary wireless communication system not only supports the GSM system, but also supports the digital communication system (DCS) system, personal communication services (PCS) system, and the WCDMA system. The frequency bands of the DCS system, the frequency bands of the PCS system and the frequency bands of the WCDMA system are located at 1710-1880 MHz, 1850-1990 MHz, and 1920-2170 MHz, respectively. Because conventional antenna is adapted for single frequency band application or dual frequency band applications only, it is obvious that the limited frequency bandwidth of the conventional antenna can not be adapted for the GSM system, the DCS system, the PCS system, and the WCDMA system simultaneously.
Therefore, it is a major concern of the present invention to develop a multiple frequency band antenna with a larger frequency bandwidth for obviating the drawbacks encountered by the prior art.